1. Field of the Invention
The present invention relates to mounting means for a power unit such as an engine, and more specifically, to hydraulic mounting means for a power unit. More specifically, the present invention pertains to mounting means for a power unit which is of a type including a pair of hydraulic mounting units located at the opposite sides of the output shaft of the power unit and having hydraulic chambers connected together by conduit means.
2. Description of the Prior Art
For mounting power units such as vehicle engines on supports such as vehicles bodies, conventional techniques use rubber mounts having spring coefficients which increase as the loads increase. Such rubber mounts are disadvantageous in that the natural frequencies of the mounting means become close to the frequencies of the secondary vibrations of the power units. In order to eliminate the problem, a proposal has been made by the Japanese patent application No. 57-41921 field on Mar. 17, 1982 and disclosed for public inspection on Sept. 26, 1983 under the disclosure No. 58-161617 to use hydraulic mounting units. According to the proposal, the mounting units are provided at the opposite sides of the power unit and each of the mounting units has an upper and lower hydraulic chambers separated by a flexible partition member which is connected with a leg member provided on the power unit. The upper chamber of one mounting unit is connected with the lower chamber of the other mounting unit through a conduit. Similarly the upper chamber of the other mounting unit is connected with the lower chamber of the one mounting unit. The proposed arrangement can provide an increased resistance to rolling oscillations of the power unit with a relatively soft support against vertical vibrations.
It should however be noted that the proposed arrangement is disadvantageous in that, due to the increased rigidity against the rolling movements, it becomes difficult to isolate torque variations of the power unit. Therefore, the torque variations of the power unit are transmitted to the support without being isolated by the mounting units.
The U.S. Pat. No. 2,705,118 issued to M. G. Beck on Mar. 29, 1955 discloses a resilient mounting system which includes a pair of mounting units located at the opposite sides of the output shaft of an engine and each having a rubber mount provided at a lower side with a hydraulic chamber. The hydraulic chamber of one mounting unit is connected with the hydraulic chamber of the other mounting unit through a conduit which may be provided with an orifice so that torsional vibrations are absorbed by the damping effect of the orifice. As described in the U.S. Pat. No. 2,705,118, the proposal is intended to provide an essentially rigid support insofar as the vertical translation is concerned but to make the torsional or rolling spring coefficient of the mounting system relatively small. The inventors have found however that it is not easy to decrease the rolling spring coefficient in this type of mounting system, because the rolling spring coefficient changes significantly in accordance with the frequency of the engine torque variations. It is understood that this tendency is produced due to the resonance of the fluid pressure in the conduit and has the following characteristics.
(1) Under a low frequency region, it is possible for the hydraulic fluid to move through the conduit connecting the hydraulic chambers of the mounting units so that the rolling spring coefficient is substantially equal to the static spring coefficient of the system. Thus, the rolling spring coefficient decreases in response to an increase in the frequency.
(2) As the frequency reaches a certain value fa, the rolling spring coefficient shows a minimum value and then increases relatively rapidly as the frequency increases beyond the aforementioned certain value. This tendency is understood as being produced by the fact that the inertia of the hydraulic fluid tends to prevent the flow through the conduit. At a second value fe of the frequency, the rolling spring coefficient becomes equal to the static spring coefficient of the system wherein the communication by the conduit is interrupted.
(3) The rolling spring coefficient increases further as the frequency increases beyond the aforementioned second value to a peak value which occurs at a third value fn of the frequency.
(4) Then, the rolling spring coefficient decreases as the frequency increases beyond the third value fn and finally approaches the static spring coefficient of the system wherein the communication through the conduit is interrupted.
It will therefore be understood that, in the mounting system as disclosed by the U.S. Pat. No. 2,705,118, it is impossible to maintain the rolling spring coefficient to a desirably low value. It should further be noted that the third value of the frequency at which the peak value of the rolling spring coefficient appears substantially corresponds to the engine idling speed or a speed slightly higher than the idling speed.